An optical wedge is a wedge-shaped light guide configured to transmit light between a first light interface located at an end of the light guide and a second light interface located at a major face of the light guide via total internal reflection. Light input into the first light interface within a suitable range of input angles propagates through the optical wedge until the critical angle of internal reflection is reached, thereby allowing the light to be transmitted out of the optical wedge through the second interface. Depending upon the design of a particular optical wedge, the first light interface may be either at a thin end or a thick end of the optical wedge. In either case, the internal reflection of light within the optical wedge allows light to fan out to a desired beam size within a relatively small volume of space, and therefore may permit the construction of a relatively compact optical system compared to a similar system without an optical wedge.
Surface roughness may negatively impact the performance of an optical wedge, as such roughness may result in light leakage and may impact image quality. A surface roughness of 2 nm RA (Roughness Average, defined as an arithmetic average of an absolute deviation from a mean surface level) or less may be desirable in some applications to avoid such problems. Further, poor dimensional accuracy of an optical wedge can lead to shearing, splitting, or ghosting of an image.